Bone is a composite material composed of impure hydroxyapatite, collagen, and a variety of noncollagenous proteins, as well as embedded and adherent cells. Bone can be processed into an implantable material, such as an allograft, for example, by treating it to remove the cells, leaving behind the extracellular matrix. The processed bone biomaterial can have a variety of properties, depending upon the specific processes and treatments applied to it, and may be combined with other biomaterials to form a composite that incorporates characteristics of both bone and the other biomaterials. For example, bone-derived materials may be processed into load-bearing mineralized grafts that support and integrate with the patient's bone or may alternatively be processed into soft, moldable or flowable demineralized bone biomaterials that have the ability to induce a cellular healing response.
The use of bone grafts and bone substitute materials in orthopedic medicine is well known. While bone wounds can regenerate without the formation of scar tissue, fractures and other orthopedic injuries take a substantial time to heal, during which the bone is unable to support physiologic loads. Metal pins, screws, and meshes are frequently required to replace the mechanical functions of injured bone. However, metal is significantly stiffer than bone. Use of metal implants may result in decreased bone density around the implant site due to stress shielding. Furthermore, some metal implants are permanent and unable to participate in physiological remodeling.
Bone's cellular healing processes, using bone tissue formation by ostoblast cells coordinated with bone and graft resorption by osteoclast cells, permit bone grafts and certain bone substitute materials to remodel into endogenous bone that is almost indistinguishable from the original. However, the use of bone grafts is limited by the available shape and size of grafts and the desire to optimize both mechanical strength and degradation rate. Variations in bone size and shape among patients (and donors) also make bone grafts a less optimal substitute material. Bone substitute materials and bone chips are quickly remodeled but cannot immediately provide mechanical support. In contrast, cortical bone grafts can support physiological stresses but remodel slowly.
Thus, it is desirable to have a bone substitute material for structural grafts that may be produced in larger quantities than grafts derived solely from bone and that may be fabricated into shapes without being limited by the shape of the originating tissue.
Additionally, it is desirable to have a bone substitute material that may be adapted to a desired shape during implantation.